1. Field of the Invention
The present invention relates to systems and methods for conditioning signals obtained from a sensor preferably located in a coronary vessel.
2. Description of the Prior Art
The assessment of cardiac function is a common problem in the evaluation of patients. The gold standard is the measurement of the ejection fraction (EF or LVEF) by echocardiography. The ejection fraction is the amount of blood pumped out of a ventricle during each heart beat. The EF evaluates how well the heart is pumping. The normal value is 65 percent (with a typical range being 50-70 percent). Lower values indicate ventricular dysfunction. The normal left ventricle has an average volume of 100 ml. Each heart beat ejects approximately 70 ml, which results in an EF≈70%. Noninvasive measurement of ejection fraction by echocardiography requires expertise and (expensive) equipment that is not always available. Further the patient must travel to the hospital.
A similar measurement of cardiac contractility, the ability of the myocardial muscle to shorten itself, is of interest to monitor continuously without advanced technical support. Invasive measures of myocardial contractility are performed by left heart catheterization, angiography and radionuclide ventriculography. None of the techniques described above allows for trending when the patient is at home.
It has been known by clinicians that there is a direct relationship between the systolic time intervals (particularly PEP/LVET, i.e., Pre-Ejection Period/Left Ventricular Ejection Time) and the left ventricular ejection fraction. (See, for instance: Hanna L, et al., “Non-invasive ejection fraction monitoring: A comparison of the impedance method to the radionuclide cardiography,” Anesth 1989; 69: A308.)
The timing relation PEP/LVET=STR (Systolic Time Ratio) has been found to be inversely related to cardiac contractility. The normal value is 0.35. A higher value indicates impaired contractility. The ratio PEP/LVET was first used by Weissler more than 35 years ago to noninvasively calculate the cardiac ejection fraction (see Garrad C L Jr, Weissler A M, and Dodge H T, “The relationship of systolic time intervals to ejection fraction in patients with cardiac disease.” Circulation 1970; 42: 455-462.)
The ejection fraction is determined by the Weissler method as follows:EF=1.125−1.25*(PEP/LVET)                where EF=ejection fraction,        PEP=pre-ejection period, and        LVET=left ventricular ejection time.        
Capan validated a similar method (Capan L V, Bernstein D P, Patel K P et al. “Measurement of ejection fraction by bioimpedance.” Crit Med 1987; 15: 402.)
Ejection fraction is determined by the Capan method as follows:EF=0.84−0.64*(PEP/LVET)
Tracing of timing parameters in the heart interval provides important clinical measures that otherwise has to done at the hospital. This can be done continuously by an implanted device. Continuous monitoring of cardiac function makes it possible to trace the outcome of given therapies, including implanted devices and drugs. The measured values can also be used to control the device stimulation therapy in a closed loop manner.
One scenario is to transfer the monitored parameters stored in the device to the hospital by Internet. Medical actions can then be taken if the cardiac parameters pass predetermined limits.
It would be of great benefit to the patient to be able to continuously measure the PEP and LVET in the implanted device. Similarly, trending of contractility and EF can be performed by using the relation PEP/LVET. Long time trending of EF is important as the EF value varies in both the short and long terms, from beat to beat, minute to minute, hour to hour, and day to day.